1. Field of the Invention
The present invention relates to a cylinder head that enables for a high engine thermal efficiency to be maintained while keeping NOx levels low with improving the combustion performance of a lean burn gas engine.
2. Description of the Related Art
In an internal combustion engine such as a gas engine or the like, a seat ring is mounted on an aperture of an intake port that communicates with a combustion chamber, and there is provided a seat surface that is in contact with an intake valve that opens and closes this aperture.
An example of the configuration of a conventional seat ring is shown in FIG. 11. This seat ring (referred to below as a standard seat ring) has an annular configuration, the inner diameter φA of which is concentric with the outer diameter F. Namely, the inner diameter φA and the outer diameter F have the same center a.
In a gas engine the combustion of a fuel-air mixture of fuel gas and air is carried out by flame propagation. Accordingly, the active use of an air flow provides an improvement in thermal efficiency by shortening the combustion period.
However, because the inner diameter φA is concentric with the outer diameter F in a standard seat ring, in a gas engine fitted with a standard seat ring the swirl ratio is small and it is difficult to improve the performance of a combustion that makes use of air flow.
Furthermore, the latest diesel engines are designed such that a relatively high output can be obtained in proportion to the size of the engine. Namely, it is required that the engine is a high brake mean effective pressure (BMEP) engine. Consequently, in order to secure the necessary quantity of air for combustion to proceed, there is a tendency to employ a high pressure ratio type of supercharger, and to design the intake and exhaust ports of the cylinder head with a large cross-sectional area and with a configuration that has few bends and the like. In ports with this type of configuration the flow coefficient is high but a strong swirl flow is not generated.
In contrast, a pilot oil ignition gas engine has been developed that aims to provide cleaner exhaust through perfect combustion, a high output, and a high thermal efficiency. This pilot oil ignition gas engine aims to keep production costs in check by using the same principal components as those used in a diesel engine.
Because a cylinder head of this pilot oil ignition gas engine is the same as that used in a diesel engine, it has a high flow coefficient, however, the swirl ratio is zero and no improvement in the combustion performance by making use of air flow can be expected.
Accordingly, in the development of this pilot oil ignition gas engine, the suppression of the flow coefficient to a minimum at the same time as the swirl ratio is improved is desired in order to improve thermal efficiency.
The present invention was conceived in view of the above circumstances and it is an object thereof to provide a cylinder head that enables the conflicting elements of the flow coefficient being maintained as well as the swirl ratio being strengthened to both be achieved, and to thereby obtain an improvement in thermal efficiency.